1. Field of the Invention
This invention relates to integrated circuits, and, more particularly, to fuses useful in programming integrated circuits.
2. Prior Art
In programmable read only memories (PROMs), various materials are used for fuses, which are melted or "blown" to program the integrated circuit device. Such materials include nickel-chromium alloys, polysilicon and refractory metal silicide. Also as part of the circuitry of the device, some PROMs typically have Schottky diodes formed by a metal contact to the semiconductor substrate on which the integrated circuit is manufactured. Two patents disclosing platinum silicide fuses and Schottky diode devices have been issued to William L. Price and assigned to the present assignee; the first patent is U.S. Pat. No. 4,042,950, entitled "Platinum Silicide Fuse Links for Integrated Circuit Devices", issued Aug. 16, 1977, and the second, U.S. Pat. No. 4,135,295, entitled "Process of Making Platinum Silicide Fuse Links for Integrated Circuit Devices", issued Jan. 23, 1979. These patents disclose a method and structure of manufacturing platinum silicide fuses on a silicon substrate. The final product has a platinum silicide fuse having one end connected by a first metal interconnection line to other parts of the integrated circuit. The other end of the fuse is connected by a metal interconnection line to a Schottky diode, which is formed by a platinum silicide contact to the lightly doped substrate below.
The required metal interconnection between the fuse and the Schottky diode generates two problems. First, it limits the minimum area need to design a fuse cell (a fuse and a Schottky diode). Second, the manufacturing steps needed to metallize a Schottky diode degrade its forward and reverse electrical characteristics. This happens even if the metallization is allowed to overlap the Schottky, diode to form a field plate around the diode.
In a Schottky diode fuse array, reverse Schottky leakage is important because during the programming of a fuse every Schottky diode with the exception of the one associated with the fuse to be programmed is reverse-biased. Current which is leaked by the reverse-biased Schottky diodes is lost from the current available to program the selected fuse. In a large PROM single diode leakages are multiplied to quite large array current leakage, anywhere in range of 5-50 mA. In order to not lower programming yields, these leakages must be compensated for. This requires increasing the peripheral circuitry which, in turn, increases die size, an undesirable result.
The present invention is a significant improvement over this prior art.